1. Field of the Invention
The invention relates in general to human joint replacement through the use of a prosthesis, and in particular to total shoulder prosthetics.
2. Related Art
The shoulder joint is the largest joint in the human upper torso, and functions as the fulcrum for all upper torso activities. Like knees and hips, the shoulder is a heavily used joint, often called upon to support or transmit high forces. However, unlike knees and hips, shoulders also have a wide range of motion. The shoulder""s unique combination of features comes at the cost of stability. A shoulder may become problematic due to several disorders and conditions. These disorders can lead to abnormal wear similar to those suffered by the weight bearing joints of the lower extremity. Thus, replacement of portions of the shoulder that contact each other, typically the epiphysis of the humerus, or humeral head, and the epiphysis of the scapula, or glenoid, is a fairly common procedure.
Given the frequency with which shoulder arthroplasty, the replacement of the humeral head and glenoid, is performed, surprisingly little research has been performed to design shoulder-specific prosthetics. In fact, due to similarities of the hip and shoulder joints, typical shoulder prosthetics have been little more than slightly redesigned hip prosthetics. However, as previously mentioned, human hips do not have the same range of motion as human shoulders, and the anatomical design of the femur, the primary hip bone, is different from the humerus, the primary bone of the shoulder. These differences lead to significant deficiencies in typical shoulder prosthesis designs.
Traditional shoulder replacement involves removal of the head of the humerus, extensive reaming of the diaphysis of the humerus, and insertion of a long (typically ten or more centimeters) metallic stem into the diaphysis to increase overall prosthetic leverage and stability. The inserted stem is then cemented into place, effectively immobilizing the rod and strengthening the connection between the rod and the humerus. The top of the inserted rod may be designed to accept an artificial humeral head, and the mounting of such a head completes the reconstruction.
Although long prosthetic stems increase leverage and stability, long prosthetic stems also cause stress shielding of the proximal humeral metaphysis. A long stem causes stress to bypass the metaphysis, which results in bone loss and associated loss of function. An example of this is loss of the greater tuberosity with the resultant loss of rotator cuff function.
This loss of proximal bone also creates difficulties with revision procedures. Cement in the metaphysis and the diaphysis of the humerus can be difficult to remove during revision of prosthetic stems. Removal of cement, combined with thin and atrophic bone that results from stress shielding, makes prosthesis revision even more difficult.
In addition to stem insertion, other techniques exist for the replacement of some or all of the shoulder. For example, U.S. Pat. No. 4,550,450 (xe2x80x9cthe ""450 patentxe2x80x9d) teaches replacement of a portion of the humeral head with high density plastic, ceramic, or metallic surfaces, without the use of a rod in the diaphysis. However, the ""450 patent uses a small trapezoidal keel which is inserted into the metaphysis. To accommodate the keel and properly anchor it in place, the metaphysis must be reamed and the keel must be cemented into place.
While the ""450 patent allows for humeral head replacement without the insertion of a long rod into the diaphysis, the ""450 patent has several deficiencies. For example, the ""450 patent does not allow the use of modular components. Without modularity, a surgeon is limited in the choice of humeral heads and stem sizes. The ""450 patent also has a centered head, rather than an offset head. A centered head does not cover the metaphysis as well as an offset head. In addition, the ""450 patent requires the use of cement to hold the stem in place. Introducing cement into the metaphysis can result in significant bone loss when revising this type of prosthesis, due to cement forced into bone pores during implantation.
It is therefore an object of the invention to provide an improved shoulder prosthesis.
Another object of the invention to allow the use of a modular prosthetic humeral head in conjunction with such a prosthesis.
It is a further object of the invention to allow implantation of support structures for such a humeral head while reducing or eliminating the need for cement or other bonding materials.
Yet another object of the invention is to provide adequate prosthetic strength without reaming or insertion of rods or other devices into the diaphysis.
A further object of the invention to provide an increased range of motion by more naturally matching prosthetic and natural humerus and glenoid shapes.
In a preferred embodiment, the invention provides these and other objects as a result of a reconceptualization of total shoulder arthroplasty. While known total shoulder prostheses have been designed using principles developed for prosthetic hips, the present invention has been designed specifically for shoulder arthroplasty. In addition, the present invention also allows for fracture repair using techniques and apparatuses similar to those used in an arthroplasty. The preferred embodiment includes a new humeral head replacement for osteoarthritis and other degenerative disorders, a new total shoulder prosthesis, a new fracture component and revision stem, and a new method of assembling, inserting, and attaching the prosthesis.
The humeral head replacement of the preferred embodiment includes an implant that allows the attachment and removal of modular artificial humeral heads. The use of modular heads allows customization of the prosthesis for each patient. Such an implant also allows head replacement in the event a previously installed artificial head wears out, becomes deformed, or otherwise requires replacement.
In addition, the preferred implantation method allows for less bone loss when and if the prosthesis needs to be revised. To help reduce bone loss during revisions, the implant is preferably designed to obviate the need for cement when installed. Use of a non-cemented fixation technique is desirable because cement can reduce patient life expectancy due to the toxicity of some cements.
In a preferred embodiment, the implant may be fixated by a press fit of the prosthesis into the humeral metaphysis. Autogenous bone graft may be used to improve the fit and encourage in-growth into the porous coat of the prosthesis. In an alternative embodiment, a mixture of autogenous graft, plaster of Paris and bone morphogenic protein or another osteogenic substance may be used to increase in-growth into the implant.
Further aspects of the invention include instruments that allow implantation of the new device. Instrumentation is kept simple in an attempt to keep the number of instruments at a reasonable level.
In a further aspect of the invention, the present invention may accept modular heads that support different head neck angles, as well as offset heads. These two design features help maximize the amount of contact between a porous coating on the prosthesis and the humeral metaphysis. Offset heads may include guides which are marked to allow accurate reproduction to be made on the implant.
The porous coating of the implant and head encourages metaphyseal bone to grow around and fuse with the prosthesis. Bone in-growth into the porous coating fuses the prosthesis with the surrounding bone, fixing and stabilizing it to the bone without cement. In addition, bone and prosthesis fusion may constitute a barrier to the migration of polyethylene particles into the humeral metaphysis and shaft. This may prevent these particles from causing osteolysis of the bone.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings, in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention.